1. Field of the Invention
The invention relates to a magnetic trip mechanism for a circuit breaker and, more particularly, to a magnetic trip mechanism including a movable core. The invention also relates to a circuit breaker including a magnetic trip mechanism, such as a solenoid.
2. Background Information
Circuit breakers are well known in the art. Examples are disclosed in U.S. Pat. Nos. 4,503,408; 5,927,484; 6,366,187; and 6,768,404, which are incorporated by reference herein.
Molded case circuit breakers, for example, typically include separable contacts, an operating mechanism, and a trip unit, which are mounted inside of a molded plastic insulative housing.
A common type of magnetic trip device for a trip unit is a solenoid, which includes a stationary core through which passes the current in the protected circuit. This current creates a magnetic field. When there are relatively very high instantaneous currents, such as those associated with a short circuit, the magnetic field intensifies. The magnetic trip device may include a plunger assembly having a movable core and a plunger tab, which engages a trip bar lever of the operating mechanism. The plunger assembly is partially disposed within the stationary core. Typically, a spring provides a limited force biasing the movable core away from the stationary core and preventing the plunger from engaging the trip bar lever. When a short circuit occurs, the current in the stationary core creates a magnetic field strong enough to overcome the movable core spring, thereby allowing the movable core to move toward the stationary core and causing the plunger to engage the trip bar lever.
U.S. Pat. No. 6,768,404 discloses a movable core positioning member having a hook and an opening. The hook engages the wall of a plunger assembly support structure. The movable core passes through the opening of the positioning member, which retains the movable core within a slot of the plunger assembly support structure. Otherwise, without the movable core positioning member, the plunger assembly may become misaligned relative to the stationary core or the trip bar lever.
As shown in FIG. 1, the plunger assembly 28 of U.S. Pat. No. 6,768,404 includes a movable core 30 having a cylindrical portion 32, a coil spring 34 and a molded plunger member 35. The movable core 30 is preferably a solid metal cylinder. The molded plunger member 35 includes a resilient C-shaped opening 37 that may be suitably expanded to be disposed about and grasp the movable core 30. The coil spring 34 is also disposed about the movable core 30. The plunger assembly 28 is disposed within a cavity 29 of a plunger assembly support structure 50. However, the molded plunger member 35 may rotate about the movable core 30 until it assumes the position shown in phantom line drawing in FIG. 1. If the molded plunger member 35 rotates to this position after the circuit breaker (not shown) has tripped (e.g., which may be the position in which the manufactured circuit breaker is shipped), then the plunger tab 36 (shown in phantom line drawing) prevents the trip bar 21 from rotating (as shown at arrow 49) back from a tripped position to a reset position. This could keep the circuit breaker from resetting.
Also, if the circuit breaker (not shown) is latched (i.e., open or closed) and the molded plunger member 35 rotates to the position shown in phantom line drawing, then the circuit breaker may trip somewhat prematurely.
Accordingly, there is room for improvement in circuit breakers and magnetic trip mechanisms including a movable core.